The invention relates to a method for operating an electronically controllable braking system for a motor vehicle, which includes a hydraulic brake-pressure generator which generates the brake pressures for the wheel brakes of the vehicle, wherein in the hydraulic connection to the brake-pressure generator a valve arrangement has been assigned to each wheel brake in order to adjust the brake pressures demanded for the wheel brakes in multiplex mode.
A braking system of such a type is known from, inter alia, WO 2010/091883 A1. This braking system exhibits a piston/cylinder system driven by an electric motor, which via hydraulic lines is connected to at least two wheel brakes, whereby in each instance a 2/2-way valve has been assigned to a wheel brake and the hydraulic connecting lines between the wheel brakes and the piston/cylinder system are capable of being closed, optionally separately or jointly, by means of the 2/2-way valves, so that a pressure is capable of being regulated in the wheel brakes in succession in multiplex mode. In this braking system the flow resistances have been dimensioned in such a way that the piston speed of the piston/cylinder system determines the pressure-reduction gradient and pressure-buildup gradient in each wheel brake, so that during the reduction of pressure and build-up of pressure the piston speed is regulated or controlled in a manner depending on the pressure/ volume characteristic of the wheel brakes. With the aid of an additional pressure model, the pressure in the wheel brakes is constantly included in the calculation. As soon as the target pressure for a wheel brake has been attained, the respective 2/2-way valve is closed. The piston of the piston/cylinder system then moves on, in order to operate the remaining wheel brakes. In the last wheel brake to be operated the pressure control is undertaken via the piston travel, which was calculated previously from the pressure/volume characteristic. After this, the 2/2-way valve of the last wheel brake can be closed.
Since in the case of the braking system known from WO 2010/091883 A1 the reduction of pressure and the build-up of pressure are regulated or controlled in a manner depending on the pressure/volume characteristic of the wheel brakes, there is, in fact, the advantage of being able to calculate the pressure in the wheel brakes with the aid of a pressure model, so that a saving can be made on corresponding pressure sensors. However, there is the disadvantage that, by reason of the dependence on the piston travel or the piston speed of the piston/cylinder system, priority cannot always be given to the build-up of pressure or reduction of pressure at a particular wheel brake. This can impair the control quality in the case of an anti-lock braking system (ABS) or electronic stability program (ESP), and may consequently have a critical effect on the braking distance and the stability of the vehicle.